The resistance material is either a resistance wire or a bit of carbon, the latter being the better material, although both are good. The fusible material is some alloy melting at a low temperature. Lead, tin, bismuth, and cadmium can be combined in such proportions as will enable the alloy to melt at temperatures from 140° to 180° F. Such an alloy is a solder which, at ordinary temperatures, is firm enough to resist the force of powerful springs; yet it will melt so as to be entirely fluid at a temperature much less than that of boiling water.

Fig. 224. Heat Coil
[View full size illustration.]

Heat Coil. Fig. 224 shows a practical way of bringing the heating and to-be-heated elements together. A copper spool is wound with resistance wire. A metal pin is soldered in the bore of the spool by an easily melting alloy. When current heats the spool enough, the pin may slide or turn in the spool. It may slide or turn in many ways and this happily enables many types of arresters to result. For example, the pin may pull out, or push in, or push through, or rotate like a shaft in a bearing, or the spool may turn on it like a hub on an axle. Messrs. Hayes, Rolfe, Cook, McBerty, Kaisling, and many other inventors have utilized these combinations and motions in the production of sneak-current arresters. All of them depend on one action: the softening of a low-melting alloy by heat generated in a resistance.

When a heat coil is associated with the proper switching springs, it becomes a sneak-current arrester. The switching springs always are arranged to ground the line wire. In some arresters, the line wire is cut off from the wire leading toward the apparatus by the same movement which grounds it. In others, the line is not broken at all, but merely grounded. Each method has its advantages.

Complete Line Protection. Fig. 225 shows the entire scheme of protectors in an exposed line and their relation to apparatus in the central-office equipment and at the subscriber's telephone. The central-office equipment contains heat coils, springs, and carbon arresters. At some point between the central office and the subscriber's premises, each wire contains a fuse. At the subscriber's premises each wire contains other fuses and these are associated with carbon arresters. The figure shows a central battery equipment, in which the ringer of the telephone is in series with a condenser. A sneak-current arrester is not required at the subscriber's station with such equipment.

Assume the line to meet an electrical hazard at the point X. If this be lightning, it will discharge to ground at the central office or at the subscriber's instrument or at both through the carbon arresters connected to that side of the line. If it be a high potential from a power circuit and of more than 350 volts, it will strike an arc at the carbon arrester connected to that wire of the line in the central office or at the subscriber's telephone or at both, if the separation of the carbons in those arresters is .005 inch or less. If the carbon arresters are separated by celluloid, it will burn away and allow the carbons to come together, extinguishing the arc. If they are separated by mica and one of the carbons is equipped with a globule of low-melting alloy, the heat of the arc will melt this, short-circuiting the gap and extinguishing the arc. The passage of current to ground at the arrester, however, will be over a path containing nothing but wire and the arrester. The resulting current, therefore, may be very large. The voltage at the arrester having been 350 volts or more, in order to establish the arc, short-circuiting the gap will make the current 7 amperes or more, unless the applied voltage miraculously falls to 50 volts or less. The current through the fuse being more than 7 amperes, it will blow promptly, opening the line and isolating the apparatus. It will be noted that this explanation applies to equipment at either end of the line, as the fuse lies between the point of contact and the carbon arrester.